This invention relates to systems that can passively and remotely sense meteorological conditions from an airborne platform or from a ground station and, in particular, to a passive polarimetric microwave radiometer for detecting aircraft icing conditions.
It is a problem in the field of aircraft operations that the occurrence of supercooled liquid droplets in the atmosphere presents a significant aviation hazard in that these particles are prone to instantaneous nucleation into solid water (ice) when subjected to minor mechanical perturbations. The size of the droplets and the temperature range of concern are approximately 5 to 200 microns in diameter and from approximately xe2x88x9230xc2x0 C. to 0xc2x0 C., respectively. If the supercooled liquid droplets undergo a phase transition to ice due to contact with a control, thrusting, lifting, or other external surface of either a fixed-wing or rotary-wing aircraft, the resulting surface becomes coated with ice, thus degrading the aerodynamic qualities of the surface and ultimately leading to reduced lift and possible stall of the aircraft. Accrued ice on external surfaces of the aircraft reduces aircraft performance, including: limits climb and ceiling capabilities, reduces airspeed, increases fuel consumption, reduces control, and reduces range. Icing also chokes engine inlets, fuel and other vents; coats radio antennas, which reduces transmission range; and obscures vision by coating windscreens and sensor optics and/or radomes. The aircraft icing hazard is especially serious for rotary wing aircraft because of the large volume of air swept by the rotor blades, the varying angle of attack of the advancing and retreating blades (as the blades rotate they tend to collect ice around the entire airfoil), and the criticality of the airfoil shape to maintaining laminar flow, and hence control and lift.
The size and temperature of water droplets determine the likelihood of ice formation. Large droplets are less common than medium-sized or small droplets because they tend to spontaneously nucleate at supercooled temperatures, and thus comprise a relatively small fraction of total supercooled cloud liquid water occurrences. Small droplets are also of less significance since they are more likely to be carried around the aircraft surface by laminar boundary airflow. Small droplets are also less likely to nucleate upon impact with aircraft surfaces due to the effects of surface tension in maintaining their spherical shape. Temperatures colder than xcx9cxe2x88x9230xc2x0 C. typically cause spontaneous nucleation, wherein a rapid conversion of the water into crystalline ice takes place. Ice crystals are less of a danger since they do not adhere to aircraft surfaces as readily as nucleating supercooled liquid. Therefore, clouds comprised solely of ice crystals are not an icing hazard. Liquid water droplets can be present along with ice crystals (a so-called mixed phase condition), and are generally depleted over time by contact with ice crystals, self-glaciation, evaporation, or precipitation processes. In the mixed-phase condition, however, the liquid droplets are necessarily supercooled, and thus present an icing hazard. Such mixed phase conditions are common within convection, a condition that is often also a source of moderate to extreme turbulence. At temperatures above freezing, water droplets remain liquid upon impact with aircraft surfaces and are rapidly shed by the slipstream.
Aircraft icing has been determined to be the cause of many aviation accidents, and can be avoided if the presence of supercooled droplets in the path of an aircraft can be determined at least a few nautical miles ahead of the aircraft. Typical flight times required for evasive maneuvering range from xcx9c15 seconds (for helicopters) to a minute (for large jet aircraft). Given the typical velocities of jet aircraft (xcx9c240-550 knots), such an icing determination would be valuable at distances out to xcx9c10 nautical miles ahead of the aircraft. Moreover, any instrumentation installed for icing detection should be simple (to be reliable), low cost (to facilitate installation on a large fleet of regional carrier and general aviation aircraft), reliable (requiring little maintenance and calibration), unambiguous in warning, and require little or no interpretation by the air crew at a time when their cockpit work load is high.
One existing icing detection system is disclosed in U.S. Pat. No. 5,028,929, wherein a forward-looking airborne radar system is used for detection of supercooled liquid droplets using a dual-frequency radar scheme. The return signals of the two radar frequencies are processed by calculating a calculus derivative of the difference in attenuation between the two radar frequencies over various radar ranges to determine the liquid water density in the atmosphere at that radar range. Suitable radar frequency pairs used in this system are X-band and Ka-band. The dual-frequency radar system is active in that it requires a powerful pulsed transmitter with associated range gating electronics. Accordingly, the dual-frequency radar system is heavy, requires a significant amount of power, is costly, and is more prone to component failure than a passive system. Matched antenna gain patterns are also critical to the accuracy of the radar measurements. Moreover, the dual-frequency radar system emits signals that are potentially detectable and trackable, thus placing military aircraft operating in a hostile environment and using the dual-frequency radar system under increased threat of detection and enemy fire. However, the dual-frequency radar system has the advantage of providing more precise range information on the distance of a supercooled liquid cell from the aircraft than a passive system.
U.S. Pat. No. 5,526,676 discloses a passive microwave radiometer utilizing a tunable frequency synthesizer as a local oscillator. Included in this microwave radiometer is a method of utilizing the varying attenuation of radio signals across an atmospheric line feature to vary the range or distance to features of interest. This microwave radiometer further measures the ranging of liquid water, the intensity of emission of which varies approximately as frequency squared, by measuring the skew of a line shape profile due to the greater enhancement of emission of the high frequency side of the line relative to the low frequency side of the line. These methodologies are pertinent to measuring the range or distance to aircraft icing conditions.
The article written by I. A. Tarabukin and G. G. Shchukin, entitled xe2x80x9cDetection of Possible Aircraft Icing in Clouds by Passive-Active Radarsxe2x80x9d, was published in the Proceedings of the Specialist Meeting on Microwave Radiometry and Remote Sensing Applications, a NOAA publication, pages 381-385, June 1992. This paper describes experimental results suggesting that icing conditions can be detected in many instances using a combination of passive and active systems. The active system (radar) determines the presence and location of clouds, and the passive system (radiometer) refines the measurement of the amount of liquid water in the cloud by measuring unpolarized emission (the first Stokes parameter) at a shallow angle above the horizon. This detection scheme requires the simultaneous use of both a passive and active sensor, and thus requires significantly more instrumentation than the system proposed herein.
A passive icing avoidance system was also described in the MIAS project to operate at 37 and 89 GHz. This system relies on extremely narrow antenna beams looking forward at two horizontal angles to detect the presence of clouds by their characteristic brightness (first Stokes parameter). It is similar to the method of Tarabukin and Shchukin above. The unpolarized emission signature of these methods can be ambiguous for some meteorological conditions, however. This is shown by icing conditions and non-icing conditions having similar emission signatures as a function of elevation angle. The system does not provide a means of distinguishing supercooled liquid droplets from ice or warm liquid droplets. Moreover, the antenna size needed to synthesize the narrow antenna beams required (1-2 degrees) are impractically large for forward-looking installations on most aircraft.
Thus, there are several existing airborne system concepts proposed for the detection of the presence of conditions which could cause aircraft icing, but each of these systems incurs penalties in sensor cost or accuracy of operation that are inherent in their underlying architecture. There is no existing system or concept that has a clear advantage over other systems and the user must make a tradeoff between cost and efficacy of the system. There are no known ground-based icing detection systems.
The above described problems are solved and a technical advance achieved by the present passive polarimetric microwave radiometer, which is a simple and low cost radiometric icing detection system that operates over suitable frequency bands in the millimeter wave region of the spectrum to provide useful signatures for detecting aircraft icing conditions. A polarimetric radiometer is distinguished from a conventional radiometer by being capable of independently measuring radiation in at least the vertical and horizontal polarizations (first and second modified Stokes parameters). This passive polarimetric microwave radiometer observes along a line of sight in or near the forward direction and consists of a dual polarization radiometer that is pointed in the direction of interest, such as the projected flight path, and operates at a frequency which is sensitive to the polarizing effects of hydrometeors (water as either liquid droplets or frozen snow, ice, or graupel particles). The mathematical difference in intensity between the vertical and horizontal polarization (the second Stokes parameter) is different for liquid and ice in sign, magnitude, and behavior at viewing angles in the vicinity of the horizon. The passive polarimetric microwave radiometer can incorporate a scanning mechanism that allows it to observe over a horizontal plane around the aircraft at the flight altitude, or can scan to observe in a vertical plane ahead of the aircraft, or can operate to scan in both the horizontal and vertical directions.
In one embodiment of this passive polarimetric microwave radiometer, the difference in microwave radiation that is scattered from hydrometeors into a near-horizontal direction in at least one frequency band viewed at about 10 degrees above the horizon and having vertical polarization compared with the microwave radiation scattered from hydrometeors into the same near-horizontal direction and having horizontal polarization. The measured difference in scattered radiation for spherical water droplets is positive, but is negative for ice crystals for viewing angles above the horizon, and positive for viewing angles below the horizon. The scattered signal is strongly dependent upon both the frequency of the microwave radiation and the mean hydrometeor size. Therefore, using a plurality of microwave frequencies along with polarization yields information on both hydrometeor size and shape (which is related to phasexe2x80x94liquid or ice).
If the microwave frequencies that are utilized have different values of signal absorption as they pass through the atmosphere, then range information to the hydrometeor is also obtained. The passive polarimetric microwave radiometer can include horizontal and/or vertical scanning to determine the range to the hydrometeors and related meteorological features, to obtain the spatial distribution of hydrometeors in the atmosphere, and to determine cloud structure from tomographic and other methods. The plurality of observation frequencies used can lie on one or several atmospheric spectral line features or in a number of window regions between spectral line features, or a combination of both. Pairs of frequencies or a number of frequencies in the atmospheric spectrum can be chosen such that the signal absorptions by the atmosphere at these frequencies are equal or approximately equal so that the range sensitivity, or distance to the meteorological feature, is matched between frequencies. Several levels of equality can be chosen such that several range sensitivities are obtained. The emission signals are thus matched at the various ranges, which enables the passive polarimetric microwave radiometer to isolate the scattered signals from the emission signals, and allows direct comparisons of scattered signals that are strongly related to hydrometeor type (i.e., water phase), size, and, in the case of ice crystals, habit (shape).
The most direct method of detecting aircraft icing conditions using the passive polarimetric microwave radiometer is detection of supercooled (i.e., temperature less than 0xc2x0 C.) liquid water droplets. This can be accomplished by passively detecting the radiation naturally emitted and scattered by water droplets and ice crystals in the atmospheric spectral region at frequencies up to xcx9c1000 GHz. FIG. 5 presents the atmospheric absorption spectrum up to about 220 GHz for atmospheric conditions and altitudes conducive to aircraft icing conditions. Frequencies in the region of 80 to 150 GHz are found to produce optimal scattering signals for typical hydrometeor occurrences.
Icing conditions occur at nearly all flight altitudes, but are most common and most persistent and severe at altitudes below xcx9c5,000 meters (xcx9c600 millibars of pressure altitude). The most hazardous conditions usually occur between xe2x88x925xc2x0 C. and xe2x88x9215xc2x0 C. Colder temperatures allow relatively little supercooled liquid to exist. As can be seen in FIG. 5, the presence of cloud liquid water droplets induces changes in the spectrum that are detectable by microwave radiometers. Additionally, atmospheric spectral line features of varying absorption offer the capability of determining the range to the origin of the signal. Because hydrometeors occur as ice-phase as well as liquid-phase droplets, and because some of the liquid phase microwave observables are similar to those generated by some occurrences of ice phase, a number of measurement types may be employed to discriminate between liquid and ice phase hydrometeors and thus to unambiguously detect the presence of icing conditions. These measurements are either polarimetric or spectral, or both, in nature. The radiometer signal can originate in part from the Earth""s surface (making the incident microwave fluxes polarized and anisotropic) and in part from the cold of outer space (making the incident fluxes unpolarized and isotropic).
In addition to the airborne application described above, this invention can function as a ground-based aircraft icing detection system by scanning the region from the horizon upward and at any or all azimuth angles. Thus, for instance, locating this invention in a suitable high location at an airport or other landing site would enable one system to service all aircraft utilizing that airport or other landing site.